Over the years, Ethernet has become the most commonly used method for local area networking. The IEEE 802.3 group, the originator of the Ethernet standard, has developed an extension to the standard, known as IEEE 802.3af, that defines supplying power over Ethernet cabling. The IEEE 802.3af standard defines a Power over Ethernet (PoE) system that involves delivering power over unshielded twisted-pair wiring from a Power Sourcing Equipment (PSE) to a Powered Device (PD) located at opposite sides of a link. Traditionally, network devices such as IP phones, wireless LAN access points, personal computers and Web cameras, have required two connections: one to a LAN and another to a power supply system. The PoE system eliminates the need for additional outlets and wiring to supply power to network devices. Instead, power is supplied over Ethernet cabling used for data transmission.
As defined in the IEEE 802.3af standard, PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device. A PD is a device that is either drawing power or requesting power. PDs may be associated with such devices as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers and HVAC thermostats.
The main functions of the PSE are to search the link for a PD requesting power, optionally classify the PD, supply power to the link if a PD is detected, monitor the power on the link, and disconnect power when it is no longer requested or required. A PD participates in the PD detection procedure by presenting PoE detection signature defined by the IEEE 802.3af standard.
If the detection signature is valid, the PD has an option of presenting a classification signature to the PSE to indicate how much power it will draw when powered up. A PD may be classified as class 0 to class 4. A PD of class 1 requires that the PSE supplies at least 4.0 W, a PD of class 2 requires that the PSE supplies at least 7.0 W, and a PD of class 0, 3 or 4 requires at least 15.4 W. Based on the determined class of the PD, the PSE applies the required power to the PD.
When power is supplied to the PD, the PSE monitors its output current with respect to certain current limit thresholds, such as the maximum output current of the PSE at a short circuit condition (ILIM), and the overload current detection range (ICUT). In particular, the PSE should be able to withstand without damage the application of short circuits of any wire to any other wire within a power supply cable, if the magnitude of the current through such a short circuit does not exceed ILIM. Further, an overload condition may be detected when an output current of the PSE exceeds ICUT for a time period exceeding an overload time limit (Tovld).
In many PSEs, values of ILIM and ICUT are maintained at fixed levels in the corresponding ranges defined by the IEEE 802.3af standard. For example, the value of ILIM may be maintained at 425 mA, while the value of ICUT may be kept at 375 mA. However, to enable the PSE to operate with various types of PDs and meet various power needs, it would be desirable to make the current limit thresholds adjustable in accordance with requirements of the PDs.
Further, the IEEE 802.3af standard specifies an input current limit threshold in a PD. In particular, the standard limits an input inrush current IInrush in a PD to the 400 mA maximum. However, it would be desirable to make the input current limit threshold adjustable in accordance with the PD's requirements.